1. Field of the Invention
The present invention relates to a device for detecting the position of stacked sheet members, and more particularly to a device for detecting the upper end position of stacked sheet members.
2. Description of the Prior Art
High speed copiers, and recording devices or printers for recording output of computers (hereinafter collectively called recording apparatus) require a stacking device capable of stacking as many output sheets as possible in order to minimize the time of interruption of function. In such stacking devices, in order to stably stack an elevated number of sheet members, it is required that the uppermost face position of the stacked sheet members is constantly maintained within a determined range with respect to a sheet ejecting device of the recording apparatus. In order to meet this requirement it has conventionally been proposed, as shown in FIG. 1, to provide, at an exit 5 of output sheet members 5, a sheet stacking table 4 for stacking said sheet members 5 ejected from ejecting rollers 6 of the recording device, also to provide a photoelectric sensor consisting of a photoemitter 1 and a photoreceptor 2 in a front end position of the sheet ejecting direction to detect if a light beam 7 emitted from the photoemitter 1 is interrupted by the elevation of the upper end level of the stacked sheet members, and to lower the sheet stacking table 4, in case of such interruption, to a position where said interruption no longer takes place. However, such arrangement where the stack level detection is conducted only at the front end position thereof, may cause excessive elevation of the rear end of the stacked sheet members for example by sheet curling as shown in FIG. 2 beyond the level of the exit of sheet members to the stacking device to clog said exit 5, thereby leading to sheet jamming in the stacking device or at said exit. On the other hand if the photoelectric sensor is positioned lower than in the above-mentioned arrangement in order to avoid such jamming, a longer distance from the exit 5 to the upper stack end may result in a defective alignment of the sheet members. Also such defective sheet alignment may result if the sheet stacking table is excessively lowered because of the curling of the front end of the sheet members as shown in FIG. 3.